In the field of microfabrication represented by the manufacture of integrated circuit devices, lithographic technology enabling microfabrication with a line width of 0.1 μm or less has been demanded in recent years in order to increase the degree of integration. However, microfabrication at a subquarter micron level is said to be very difficult using near ultraviolet rays such as i-rays which are generally used as radiation in a lithography process. Therefore, in order to enable microfabrication with a line width of 0.1 μm or less, use of radiation with a shorter wavelength has been studied. As examples of such short wavelength radiation, a bright line spectrum of a mercury lamp, deep ultraviolet rays represented by excimer lasers, X rays, electron beams, and the like can be given. A KrF excimer laser (wavelength: 248 nm) or an ArF excimer laser (wavelength: 193 nm) are being given particular attention.
As a resist suitable for being irradiated with such an excimer laser, many resists utilizing the chemical amplification effect of a component having an acid-dissociable functional group (hereinafter referred from time to time to as “an acid generator”) and exposure to radiation (hereinafter referred to from time to time simply as “exposure”) have been proposed. Such a resist is hereinafter referred to from time to time as a “chemically-amplified resist”. As a chemically amplified resist, a resist containing a resin having a tert-butyl ester group of carboxylic acid or a tert-butyl carbonate group of phenol and an acid generator has been proposed (see Patent Document 1, for example). The principle of the method is as follows. The tert-butyl ester group or tert-butyl carbonate group in the resin dissociates by the action of an acid generated upon exposure, whereby the resist is provided with an acidic group such as a carboxylic group or a phenolic hydroxyl group. As a result, the exposed areas of the resist film become readily soluble in an alkaline developer.
A capability of forming a finer pattern (e.g., a fine resist pattern with a line width of about 45 nm) will be required for such a lithographic process in the future. Reducing the wavelength of a light source of an exposure apparatus and increasing the numerical aperture (NA) of a lens could be a means for forming such a pattern with a line width of less than 45 nm. However, an expensive exposure apparatus is necessary for reducing the wavelength of a light source. In addition, increasing the numerical aperture (NA) of a lens involves a problem of decreasing the depth of focus even if resolution is increased due to the trade-off relationship between the resolution and the depth of focus.
Recently, a liquid immersion lithographic method has been reported as a lithographic technique enabling solution of such a problem (for example, see Patent Document 2). In the liquid immersion lithographic process, a layer of a liquid high refractive index medium (liquid for liquid immersion lithography) such as pure water or a fluorine-containing inert liquid is caused to be present between the lens and the resist film on a substrate, at least on the surface of the resist film during exposure. According to this method, an inert gas in the light-path space for exposure light, such as air and nitrogen, is replaced by a liquid with a larger refractive index (n), for example, pure water, whereby resolution can be increased without a decrease in focal depth by using a light source with a given wavelength to the same degree as in the case in which a light source with a shorter wavelength is used or the case in which a higher NA lens is used. Since a resist pattern with a higher resolution excelling in focal depth can be formed at a low cost using a lens mounted in existing apparatuses by using the liquid immersion lithographic method, the method has received a great deal of attention and is currently being put into practice.
Although downsizing of the line width of the above exposure technology is said to be up to 45 nm hp at most, the technological development is advancing toward a 32 nm hp generation requiring further minute fabrication. Since the apparatus (liquid immersion lithographic apparatus) used for the liquid immersion lithographic method is very expensive, the method lacks practical utility in the actual semiconductor manufacturing process.
On the other hand, following the complication and demand for high integration of devices, a pattern forming method of using a thin film formed on the surface of the wall on the right and left sides of a dummy pattern as a gate electrode or the like after removing the dummy pattern has been proposed in order to reduce linewide roughness (LWR), (for example, refer to Nonpatent Document 1). In the pattern forming method proposed in the Nonpatent Document 1, the LWR is reduced by evaluating the variation of the threshold voltage of a transistor. However, the method can be used only with difficulty for forming a finer pattern.    Patent Document 1: JP-A-5-232704    Patent Document 2: JP-A-10-303114    Non-patent Document 1: International Electron Devices, Meeting Technical Digest, pp. 863-866, December 2005